For utility vehicles which work on sloping grounds, it may be required to reposition their frame with respect to the ground. This can be generally required for stability reasons, but it is particularly important for agricultural vehicles, which are driven on slopes during the treatment of vertically growing plants. In order to obtain a good effect on the plants, it is necessary to keep the frame of the travelling harvesting vehicle in a fixed position relative the vertically growing plants, irrespective of the slope they are growing on.
Conventional grape harvesters, for example, comprise a high straddler frame, which is travelled over the vines and to which a shaker and grape collecting apparatus is attached. The frame is supported on four wheels by extendable suspension means, which are operable to level the frame, irrespective of the direction and degree of slope of the ground. Most commonly, these suspension means comprise a set of hydraulic cylinders, which can be operated to lift or lower the one or other side of the frame.
Such a levelling system has been disclosed in U.S. Pat. No. 4,247,126, issued Jan. 27, 1981 in the name of Gerald L. Claxton wherein the rear and front cylinders on each side of the frame are hydraulically interconnected in order to form a master-slave system, which is operable to simultaneously extend or retract said cylinders in order to lift or lower the corresponding side of the frame. During normal harvesting operations, when no levelling action is taking place, the rear cylinders are hydraulically blocked as to maintain the rear wheels in a fixed vertical position relative to the frame, thereby forming a fixed rear axle of the vehicle.
In order to keep all wheels in firm ground engagement in spite of instant variations in the ground contour, the upper and lower chamber of the one front cylinder are permanently connected to the respective chambers of the other front cylinder, so that the retraction of one front cylinder causes the extension of the other by an equal amount. The front of the frame is still supported by the oil enclosed in the front cylinders, but the front wheels can move up and down, as if they were mounted on an articulated axle. This configuration makes the vehicle rest on its four wheels, which is particularly advantageous when all wheels are driven; otherwise the wheel which loses contact with the ground, may start to spin, so that the traction power of a portion of the drive line is lost.
Although this levelling system works well under most circumstances, it has been experienced that it is not capable of correcting the position of the frame under some rare, but very hazardous circumstances. This is the case when the high vehicle is travelling down a steep slope and one of the front wheels enters into a pit, whereby the vehicle starts to tilt sideways. One of the rear wheels even may loose contact with the ground. The operator, who wants to stabilize the vehicle, gives a command to raise the lowered side, but the system appears unable to stop the sideways movement of the vehicle and may even accelerate the same, so that it finally turns over.